This application claims the priority of Swiss patent application 2251/99, filed Dec. 8, 1999, the disclosure of which is incorporated herein by reference in its entirety.
The invention relates to a sensor according to the preamble of claim 1 as well as a method for producing such a sensor.
Typical examples for sensors of this type are humidity sensors. These comprise a polymer or ceramic layer, which lies on two interdigital electrodes. The dielectric constant of the polymer layer and therefore also the electric capacity between the two electrodes depends on the humidity content of the environmental air. Hence, the humidity content can be determined by means of a capacitive measurement.
In practice it is found, however, that sensors of this type are subject to aging. The signal of a constant parameter to be measured changes over time. This makes later recalibrations necessary and can make the sensors useless.
Hence, it is an object of the invention to provide a sensor of the type mentioned initially that is subject as little as possible to the aging processes mentioned above.
This object is achieved by the sensor of claim 1.
According to the invention, a protective layer of a gas proof, non-oxidizing material is arranged between the electrodes and the measuring layer. As it is found, this reduces or even prevents aging processes. It is assumed that the protective layer prevents an oxidation of the electrodes.
The invention is especially effective with electrodes of aluminum or electrodes of another material that is oxidized easily, such as copper, titanium, tungsten or polysilicon. Aluminum is, however, especially preferred because it is suited for production on semiconductor chips using lithographic methods.
The protective layer can consist of a noble metal, such as gold, or a non-oxidizing dielectric, in particular silicon oxide or silicon nitride. Noble metals and in particular gold have the advantage that they are suited for selective galvanic deposition on the electrodes. For a particularly good sealing, combined layers of gold and a nonoxidizing dielectric deposited thereon are possible as well.
In a preferred embodiment the sensor consists of a semiconductor chip in which an opening is arranged. The opening is spanned by a membrane and the measuring layer is arranged at the membrane. This allows a better control of the temperature of the measuring layer, in particular if a suited heating element is provided. Preferably, the measuring layer is arranged at the inner side of the membrane such that the outer side and therefore the circuit side of the semiconductor chip can be separated from the medium to be measured.
The invention is especially suited for humidity sensors, gas sensors, etc.